Plasmacytoid dendritic cells (pDC) are lymphoid dendritic cells that express high levels of interleukin 3 (IL-3) receptor (CD 123) and produce type I interferons, cytokines with antiviral and immunoregulatory effects, pDC are present in the thymus, cord and peripheral blood and T cell areas of the lymph nodes. Peripheral blood pDC migrate to lymphoid tissue and sites of inflammation. In the human thymus pDC are localized in the medulla and at the cortico-medullary junction, but it is unknown whether they play a role in normal T cell development, pDC have the capacity to produce high levels of Interferon-alpha (IFN-alpha) in response to viruses and other stimuli. However, IFN-alpha is also an essential survival factor for pDC. High levels of IFN-alpha suppress murine T and B cell development and other hematopoietic cells in vitro and in vivo and our preliminary data show that high concentrations of IFN-alpha interferes with T and NK cell development in the human thymus. Thus, IFN-alpha may be important for normal functioning of the adaptive and innate immune system at low, physiologic concentrations, while it is immunosuppressive at high concentrations. The proposed studies are designed to elucidate the role of pDC, the main producers of IFN-alpha, in normal T and NK cell development in the thymus and why the antiviral effects of IFN-alpha are overshadowed by detrimental effects on the (developing) immune system. The central hypothesis is that low, physiological levels of IFN-alpha, increase survival of thymic pDC thereby maintaining normal T and NK cell development while high levels of IFN-alpha as observed in HIV infection, impair T and NK cell (re)generation. Experiments to test the hypothesis will be performed with endogenously produced IFN-alpha as well as exogenous IFN-alpha using T and NK cell development assays in thymic organ culture and SCID-hu mouse models. Specifically we propose to: 1. To characterize the role of pDC in T and NK cell development in the human thymus. 2. To investigate the effects of high levels of IFN alpha on T and NK cell development in the thymus. 3. To investigate the effects of HIV-1 on IFN-alpha production by pDC and the potential effects on developing T cells. The present proposal represents a unique collaborative approach to elucidate the role of pDC and IFN-alpha in T and NK cell development in the thymus. Information gained from the proposed experiments will have implications for the treatment of HIV infected patients with immunomodulatory therapies, such as IFN-alpha.